1. Field of the Invention
The invention relates to a 2-roll piercing mill for producing hollow blocks from high alloy steels. The 2-roll piercing mill has guides for the rolled material that close the reforming zone transversely with respect to the rolling axis as the hollow block is pierced.
2. Description of the Related Art
The state of the art of the piercing rolling method and the metallurgical production process presently allows production of seamless pipes of good quality from high alloy ferritic, ferritic/martensitic and austenitic steels by piercing on the piercing mill. This method has some advantages by comparison with the conventional extrusion process. The advantages of the piercing rolling method result from omitting the operation step of boring out the blocks, increasing the usable weights of the blocks and reducing the eccentricity and therefore the wall thickness tolerance. A conical piercing mill having guide shoes for closing the reforming zone transversely with respect to the rolling axis is particularly suitable for piercing high alloy steels, as has already been introduced by R. C. Stiefel. This method produces higher stretch values, thereby allowing the production of relatively thin-walled hollow blocks, which is not possible to the same extent using a 3-roll piercing mill or a 2-roll piercing mill with guide disks.
However, high alloy steels have a high tendency to adhesion (that is to say to the welding or deposition) of block material onto the operating face of the guides. The material which adheres firmly to the working face of the guides (referred to as pick ups) leads to mechanical damage in the form of deep, sharp-edged, spiral ridges on the outer surface of the perforated hollow blocks that are being formed. The mechanical damage must then be removed by grinding, skinning or turning down, which entails high material losses and additional operating outlay.
A specific application of a lubricating and release agent to the guides may avoid or at least delay the production of the material deposits. However, the lubricating agent may inadvertently be transferred to the working rolls, so that slipping (slip) between the rolls and roll material occurs. The slipping may lead to the abortion of the piercing operation.
The above mentioned slip may be avoided by supplying a liquid containing an Si carbide. However, this solution requires an additional device and an additional medium.
Some trials using guide shoes made of gray cast iron GGG and GGL showed a certain suitability with regard to adhesion, but their friction and wear behavior was unsatisfactory for production operation.